The present invention relates to electro optics and more particularly to the method and apparatus for simultaneously displaying targets at different ranges. The ability to view objects at ranges of several miles and under conditions of substantial total darkness has presented many difficulties in the past. Numerous devices have evolved which detect targets under such conditions. It can be readily appreciated that in nighttime surveillance of a battlefield area, it is highly desirable to minimize the possibility of detection of the illuminating device and yet provide a display which is the same as could be obtained during normal daylight hours. Accordingly, many devices utilize an illuminant operable in the non-visible range of the electromagnetic energy spectrum such as infrared of ultraviolet. Since a laser offers a narrow spectral band of infrared radiation and spectral discrimination, i.e., it increases the reflectivity of many materials, there are devices which use a pulsed laser to illuminate the desired area of view. There are range finders which use lasers to effectively search for targets at preselected ranges. In these active-gated laser systems, the range gate interval is successively lengthened upon each successive transmittal of a laser pulse until an echo response is detected whereupon the automatic range sweeping operation is halted. Thus, the presence of a target at a particular range is detected.
While some devices relate to visual display of objects at a predetermined distance and others relate to the ascertainment of only distance to a target, still others disclose means for both visually displaying a particular scene and at the same time determining the distance to the objects therein. U.S. Pat. No. 3,495,906 to Firmin and U.S. Pat. No. 3,649,124 to Takaoka disclose such devices. Firmin periodically illuminates a field of view and uses the propagation time of reflected successive pulses to indicate distance to the object. A special form of display tube is used wherein different deflection rates are utilized for successive pulse illuminations of the target area to result in a display wherein objects located at various ranges within the target area are displaced with respect to one another. Takaoka discloses an active-gated television system which displays successive full-frame images of various ranges as a composite. Each range is displayed as a different color in the composite display to distinguish the various ranges that are being observed. In U.S. Pat. No. 3,682,553 to Kapany, successive entire frames are generated for successively different range distances and combined into a composite display to achieve a three-dimensional view of an object within the target area and the television system.
In known devices the range being examined is held constant for a particular integration time. Since a periodically pulsed illuminator is operated in sequence with a periodically gated sensor, the spacing between the illuminator pulse and the gated sensor pulse determines the particular range being observed. The depth of field viewed at a particular range is usually very narrow in order to eliminate atmospheric backscatter. Accordingly, a potential target may be within the field of view and not be detected if the range being examined happens to vary even slightly from the range wherein the potential target is disposed. As the depth of field is increased, the atmospheric backscatter also increases to seriously degrade image quality. Also, the observed interescene brightness of various targets at different ranges varies with the square of the distance to the particular object and with atmospheric conditions through which they are viewed.
There is a need for a low-tight level device that will provide surveillance of a wide range and yet provide uniform displayed target brightness. As opposed to providing a device for viewing a first range and then a second range or a composite formed from the stored range increments, there is a need for a device which continually scans a wide path of ranges to provide a visual image.
Accordingly, it is an object of the present invention to provide a low tight level active device which provides a view of a wide depth of field with minimal image deterioration due to backscatter.
It is a further object of this invention to provide a low light level active device which displays a wide depth of field wherein the image intensities are constant over the depth of field.
It is yet another object of this invention to provide a low light level active device which displays a wide depth of field wherein the image intensities are constant over the depth of field.
It is yet another object of this invention to provide an active-gated device wherein a plurality of ranges are scanned and displayed during a single integration period of a recording device to thereby produce a continuing visual display of an entire wide depth of field.
Another object of this invention is to provide a displayed image under low or adverse ambient viewing conditions which closely simulates that which occurs with natural illumination.
Yet another object of this invention is to provide for target enhancement or suppression at a selected range interval or intervals while simultaneously displaying all other ranges of interest.
A final object of this invention is to provide a device to illuminate a scene with pulsed illumination and observe the same scene with a gated light amplifier in such a manner that the time spacing between illuminator and light amplifier pulse pairs is varied in a systematic manner such that all ranges of interest are viewed by the light amplifier within a single integration period of a recording device.